The general purpose of this project is to study the immune responses in man and animals to natural and experimental rickettsial infections. Presently we are 1) examining the protective effect in mice of human antibodies against Coxiella burnetii, 2) developing new criteria for characterization of recrudescent epidemic typhus fever (Brill-Zinsser disease), and 3) examining the decay of immunity to spotted fever and typhus fever infections in immunized guinea pigs.